In this video we're gonna look at gas stoichiometry or the stoichiometry of reactions that involve
gas. This isn't really a new concept it's just adding all of the things we've learned
about gas and calculating the number of moles, volume, and pressure into stoichiometry problems.
If we think about the reaction of propane with oxygen to give us carbon dioxide and
water, we can see that all of those involve gas. We take one mole of propane and react
it with five moles of oxygen gas to give us three moles of carbon dioxide gas and four
moles of water gas, as steam. Now we can do this calculation but since these are gases
not only can we determine moles of reactance to products and also the masses we can use
ideal gas law PV=nRT use the number of moles and calculate volume, pressure, and sometimes
even temperature. We can also consider the fact that this is a partial pressure problem,
because at any one point, we have all of these different gases in our container, if we had
a closed container. We can see that we could also use Dalton's law, total pressure equals
the sum of the partial pressures. Let look at a problem. Complete decomposition of a
pure sample of potassium chlorate produced 2.16 g of potassium chloride and oxygen gas.
What is the volume of oxygen produced at STP? Well when we look at our problem we see that
it's giving us the reaction. It's going to be the decomposition of potassium chlorate
going to potassium chloride and oxygen. So we can draw our reaction, potassium chlorate
decomposes to give us potassium chloride plus oxygen. Now since this is the stoichiometry
it's important that we balance this equation. Notice that the equation is not balanced and
so we're going to add components to balance it. Here I'm just going go ahead and put them
in because what we're gonna focus on is our formula. So it actually takes two moles of
potassium chlorate to produce two moles of potassium chloride and three moles of oxygen.
Now in our question, it gave us 2.16 grams. Not to go from grams to moles of oxygen first
we're going to have to go moles of potassium chloride and then we're gonna have to go to
moles of oxygen. And then if we had to go back we'd go grams. So let's look at this
first step so 2.16 g potassium chloride we're gonna turn that into moles of potassium chloride
means we're gonna have to divide by the molecular weight 74.55 g of potassium chloride per mole
of potassium chloride. Cancel out the grams, doo our calculation and we'll see that we
are going to come out with 0.0290 moles. Then when we'll take our moles of potassium chloride
and turn them into moles of oxygen. So 0.029 moles of potassium chloride and we're gonna
turn that into moles of oxygen. For this one we're going to need to use are stoichiometric
coefficients. Looking at the balanced equation we see we have two moles of potassium chloride
and three moles of oxygen. We cancel out our moles and we'll come out with 0.0435 moles
of oxygen. So now we have moles of oxygen but remember that the question is not asking
for moles of oxygen but the volume. It's important that you always read the question and make
sure that you're answering the right problem. The problem here is that we have to get to
the volume. We know how to get to the volume if we use the ideal gas law PV=nRT. We rearrange
to get the volume by itself: V equals nRT over P. We have the number of moles we just
calculated that. We have R, that's our gas constant. Where is T temperature and P pressure?
In our question it told us at STP. Remember that STP is standard temperature and pressure.
So T is equal to 0°C or 273.15 Kelvin and P equals 1 atm. Now we can see that plug into
the formula and we'll be able to calculate our term. So once we plug everything in you'll
see that we have 0.0435 moles times the constant for R times 273.15 Kelvin that's the standard
temperature divided by 1 atm, the standard pressure. Do this calculation and we see that
our volume is going to be 0.975 L. Double check all of your units: moles, atmospheres
and Kelvin giving us liters. Students often have problems with gas stoichiometry. You
just need to take it slow. Take the problems step by step. Make a plan for solving the
problem. There are a few steps to this. First make sure you know what the question is asking
for. Then determine what is given and what can be determined from the data in the question.
Make a plan before you start doing the cancellations. Implement your plan and check for errors at
each step. Reflect on the answer. Does it make sense? With the problem that we did we
had 0.0435 moles of oxygen and it came out to 0.975 liters. Does that make sense? Do
we have anything to compare it to? Well we know that one mole of any gas at STP is 22.4
L, so therefore 0.043 moles being about 1 L makes sense. Many of the calculations will
make sense if you just think about what the answer is and is it even the right magnitude.
Again make sure you know what the question is asking for. Determine what is given and
what can be determined from the data in the question. Make a plan. Implement your plan.
And reflect on the answer to make sure it makes sense. Practice these, step by step,
and become confident.